In packaging bags made of synthetic resin films that have been used for sealing its contents, such as food products requiring heat treatment, the following measures have been taken to remove the contents after the heat treatment of the packaging bag having the contents inserted therein.
1. A sealing agent having a low melting point is provided in advance in a zone where the packaging bag is to be opened and other zones are sealed with a constant strength. If the internal pressure is increased when the packaging bag is heated, the zone provided with the sealing agent having a low melting point is opened.
2. A portion of the packaging bag is not sealed. If the internal pressure is increased when the packaging bag is heated, vapors present inside the packaging bag escape to the outside thereof through the zone which is not sealed.
3. Vapor holes are provided in the packaging bag and a tape coated with a sealing agent having a low melting point is affixed onto the vapor holes. If the internal pressure is increased when the packaging bag is heated, the sealing agent having a low melting point is melted, the tape peels off, and the vapors present inside the packaging bag escape to the outside thereof through the vapor holes.
4. Vapor holes are provided in the packaging bag and the vapor holes are closed with a nonwoven fabric. If the internal pressure is increased when the packaging bag is heated, the vapors present inside the packaging bag escape to the outside thereof through the nonwoven fabric.
5. A zone where the packaging bag is to be opened is sealed at a low temperature and other zones are sealed at a high temperature. If the internal pressure is increased when the packaging bag is heated, the vapors present inside the packaging bag peel off the seal in the zone of the packaging bag sealed at a low temperature and escape to the outside thereof.
6. A rift is made in advance at the perimeter of a packaging bag. Immediately prior to heat treatment, the rift is broken and the vapors present inside the packaging bag escape to the outside thereof through the crack.
7. A packaging bag is produced from a two-layer film in which a surface film is laminated with a back film provided with a plurality of small openings. If the internal pressure is increased when the packaging bag is heated, the vapors present inside the packaging bag penetrate in the space between the surface film and back film through the small openings in the back film, peel the surface film from the back film, and escape to the outside.
8. A portion of a sealing zone located on the periphery of the packaging bag is provided as a narrow weak sealing zone. If the internal pressure is increased when the packaging bag is heated, the vapors present inside the packaging bag open the narrow weak sealing zone and escape to the outside.